Deoxidizing metal

ABSTRACT

A METHOD OF DEOXIDIZING METAL WHILE MAINTAINING THE CARBON CONTENT OF THE METAL AT A LEVEL ABOUT EQUAL TO OR LOWER THAN THE LEVEL PRIOR TO DEOXIDIZING. IT COMPRISES THE STEPS OF INTRODUCING HYDROCARBON DEOXIDIZER AND DILUENT GAS INTO A VESSEL CONTAINING MOLTEN METAL, DETERMINING THE EFFECT OF THE HYDROCARBON DEOXIDIZER AND DILUENT GAS UPON THE CARBON CONTENT OF THE METAL AND CONTROLLING THE PROPORTION OF HYDROCARBON DEOXIDIZER TO DILUENT GAS SO THAT THE AVERAGE RATE OF CARBON LEAVING THE VESSEL IS ABOUT EQUAL TO OR GREATER THAN THE AVERAGE RATE OF CARBON BEING INTRODUCED INTO THE VESSEL.

United States Patent 6 3,725,041 DEOXIDIZING METAL SundaresanRamachandran, Natrona Heights, Pa, as-

signor to Allegheny Ludlum Industries, Inc., Pittsburgh, Pa. No Drawing.Filed Sept. 25, 1970, Ser. No. 75,738 Int. Cl. C21c 7/06 US. CI. 75-6010 Claims ABSTRACT on THE DISCLOSURE A method of deoxidizing metal whilemaintaining the carbon content of the metal at a level about equal to orlower than the level prior to deoxidizing. It comprises the steps ofintroducing hydrocarbon deoxidizer and dilucnt gas into a vesselcontaining molten metal, determining the effect of the hydrocarbondeoxidizer and diluent gas upon the carbon content of the metal andcontrolling the proportion of hydrocarbon deoxidizer to diluent gas sothat the average rate of carbon leaving the vessel is about equal to orgreater than the average rate of carbon being introduced into thevessel.

The present invention relates to a method of deoxidizing molten metaland more particularly to a method of deoxidizing molten metal whilemaintaining the carbon content of the metal at a level about equal to orlower than the level prior to deoxidizing.

Present day metal-making; e.g., steel-making, processes often involve adeoxidizing treatment. It is highly desirable to lower the oxygencontent of a melt since oxygen dissolved in a melt can precipitate asnon-metallic inclusions which adversely atfect the properties of themetal.

Various deoxidizing methods have been employed in the past. One methodinvolves the use of highly reactive elements; e.g., silicon, titaniumand/or aluminum, which combine with oxygen in the melt to form oxidesthat subsequently separate from the melt. Sufiicient time must, however,be provided for the oxides and melt to separate. A second methodinvolves the mixing of -a predetermined quantity of carbon with the meltand a dynamic hydrogen atmosphere to control carbon boil. This method iselfective for lowering the oxygen content but often adversely afiectsthe desired low carbon content. It is disclosed in US. Pat. No.3,188,198 which issued on June 8, 1965. A third method, particularlyefiective for alloy steels with high carbon contents, involves thelowering of the partial pressure of carbon monoxide in the vessel. Alowering of the partial pressure of carbon monoxide changes equilibriumrelationships and shifts the attainable end point carbon to lower levelswithout necessitating excessive oxidation of metallic components and,thereby, frees carbon to combine with oxygen within the melt. Reductionin the partial pressure can be accomplished by reducing the pressure inthe vessel and/ or by introducing argon into the vessel. This thirdmethod, however, does not measure up to theoretical expectations as thecarbon-oxygen reaction fails to proceed to completion. From athermodynamic point of view, the system acts as if it were under ahigher pressure.

Another prior art process of considerable interest is disclosed in anarticle entitled Deoxidation Techniques for Vacuum-Induction Melting byW. F. Moore. :It appeared on pages 918-921 in the December 1963 issue ofthe Journal of Metals. The process described therein employed naturalgas which consisted of, by volume, 94.9% CH 3.2% C H 1.0% C H 0.1% C H0.6% C 0.1% H 0 and 0.1% N +CO, to deoxidize a melt. Results from theprocess were promising with regard to the degree of deoxidizing but wereunfortunately disappointing to processors who require both low carbonand oxygen contents in their steel. The natural gas injected ice anexcess of carbon into the melt and thereby, raised its carbon content.

It would appear that the major shortcoming of the process described inthe above referred to Journal of Metals article could be rectified byreducing the amount of natural gas injected into the melt. The mostobvious manner of accomplishing this would be to simply reduce thenatural gas input flow rate. This however, diminishes the mixing causedby the input of the gas which in turn reduces the degree of reactionbetween carbon that evolves from the gas and oxygen in the melt.

The present invention provides a method which eti'ectively deoxidizes amelt while controlling the carbon content at a level about equal to orlower than that which was present prior to deoxidizing. It employs atleast one hydrocarbon as a deoxidizer and at least one diluent gas. Thediluent gas enables the processor to use small amounts of hydrocarbondeoxidizer, thereby precluding excessive injection of carbon into themelt, while maintaining a gaseous injection rate which is sufiicient toinsure adequate mixing between the hydrocarbon deoxidizer and the melt.Thus, the hydrocarbon deoxidizer input rate can be lowered withoutreducing the rate of oxygen reaction with carbon, by introducing adiluent gas with the hydrocarbon deoxidizer.

It is accordingly an object of this invention to provide a method ofdeoxidizing molten metal.

It is an additional object of this invention to provide a method ofdeoxidizing molten metal while maintaining the carbon content of themetal at a level about equal to or lower than the level prior todeoxidizing.

The present invention comprises the steps of introducing hydrocarbondeoxidizer and diluent gas into a vessel containing molten metal,determining the eiiect of the hydrocarbon deoxidizer and diluent gasupon the carbon content of the metal and controlling the proportion ofhydrocarbon deoxidizer to diluent gas so that the average rate of carbonleaving the vessel is about equal to or greater than the average rate ofcarbon being introduced into the vessel.

The invention embraces the use of one or more hydrocarbon deoxidizerschosen from a wide spectrum of gase- (ms and liquid hydrocarbons andhydrocarbon containing substances as well as the use of one or morediluent gases chosen from a wide spectrum of diluent gases. Illustrativehydrocarbons and hydrocarbon containing substances are methane, ethane,propane, ethylene, water gas and natural gas. Illustrative diluent gasesare argon, nitrogen, hydrogen and carbon monoxide. Liquid hydrocarbonsand hydrocarbon containing substances require the additional step ofatomizing the liquid into the diluent gas stream. The hydrocarbondeoxidizer and diluent gas can be blown into or blown onto the top ofthe melt.

The elfect of the hydrocarbon deoxidizer upon the carbon content of amelt can be determined from the initial and final melt analysis of apreviously deoxidized melt. A comparison of the initial and finalanalysis along with the input rate and analysis of the hydrocarbondeoxidizer and/or diluent gas (diluent gas is not necessary here as thisis not necessarily a heat requiring a low carbon level) introduced intothe vessel sets forth the information necessary to determine theefficiency at which carbon, from the deoxidizer, combined with oxygen.The efiiciency at which carbon and oxygen combined tells a processorwhat the proportion of hydrocarbon deoxidizer to diluent gas should befor subsequent heats, of similar chemistry, which are to be deoxidizedunder similar conditions, e.g., similar gaseous injection rates. Forexample, a heat deoxidized with hydrocarbon deoxidizer and 20% diluentgas and having a 50% carbon-oxygen reaction etficiency indicates thatsubsequent heats should use 40% or less hydrocarbon deoxidizer and 60%or more diluent gas if the heats are of similar chemistry and are to besimilarly deoxidized.

Although the above described procedure for determining the effect of thehydrocarbon deoxidizer is far better than adequate, it does have ashortcoming. The end-point control may not be too precise due to heatvariations in analysis, temperature and efficiency of gas blowing.

An alternative process for detemining the effect of the hydrocarbondeoxidizer overcomes the shortcoming of the above described procedure.It involves calculating the rate at which carbon is introduced to thevessel and the rate at which it leaves the vessel. The carbon input ratecan be calculated from the analysis and input rate of hydrocarbondeoxidizer and diluent gas. The carbon output rate can be calculatedfrom the output rate and analysis of the gases exiting the vessel (amonitoring system can be used to analyze the exiting gases). Thecalculations enable a processor to control the carbon content of themelt by adjusting the rate at which carbon is introduced to the vessel.

The following paragraphs are exemplary of the type of reactions whichoccur during the deoxidizing method of this invention and of a methodfor calculating both the carbon input and output. Methane, CH has beenchosen as the hydrocarbon deoxidizer for purposes of illustration.

The major reactions which occur during deoxidation with methane are:

4 )+Q= H Z CH (gas)+MO=E-]CO (gasH-ZH (gas) leaving the system Nonumerical value can be set for the ratio of hydrocarbon deoxidizer todiluent gas as it can change throughout the deoxidizing treatment. Attimes, it is even desirable to complete the final stages of deoxidationwith an inert gas and without any hydrocarbon deoxidizer. The inert gaswill lower the partial pressure of carbon monoxide, change equilibriumrelationships and shift the attainable end point carbon to lower levelswithout necessitating excessive oxidation of metallic components,thereby freeing carbon to combine with oxygen within the melt. It willadditionally cause a mixing of the melt which will promote thecarbon-oxygen reaction. As a general rule the hydrocarbon deoxidizer andthe diluent gas are injected into the vessel at an average gaseousinjection rate of at least 20 cu. ft. per hour. A preferred averagegaseous injection rate is at least 30 cu. ft. per hour. Lower injectionrates are, however, embraced within this invention. A precise valuecannot be set for the minimum rate as it fluctuates with processvariables, such as the depth of the molten metal.

The invention can additionally encompass controlling of the final oxygencontent. This entails knowledge of the oxygen content prior to or atsome stage during deoxidation and calculating of the amount of oxygenleaving the system. Oxygen in the melt can be measured by an EMF cell orby chemical analysis. The amount of oxygen leaving the system can becalculated from an analysis of the gases exiting from the system andfrom the following equations:

t t 1 ft f atomic wt.

0 0 Pounds of oxygen g H O X 1 mole xygen (lbs) 2 2 360 cu. ft. averageleaving the system molecular The deoxidizing reactions can be carriedout at pressures which are at, below or above atmospheric pressure.

The reaction which leads to carburization of the bath is:

An additional carburization reaction which can occur to an intolerabledegree when there is an excessive amount of hydrocarbon deoxidizer is:

CH; (gas)=C+2H (gas) The hydrocarbon deoxidizer cracks and depositscarbon (soot) in the cooler parts of the vessel.

The carbon input and input rate can be obtained from the followingequations:

total cu. ft. of methane introduced into the vessel Rate carbon is beingpounds of carbon introduced into =introduced into the vessel the vesselPounds of carbon introduced into the vessel 1 mole X 360 cu. ft.

:time

The carbon output and output rate can be obtained from the followingequations:

total cu. ft. of

g f gi gi CH c0 and 1 mole at CO leaving the 360 cu. ft. the vesselvessel Rate of carbon pounds of earbon leaving the vesse1 leavin g thevessel wt. of gases leaving the vessel rate of oxygen pounds of oxygen+t' leaving the system leaving the system [me atomic wt. of X carbon(lbs) 1 mole atomic wt. of X carbon (lbs) 1 mole TABLE I Analysis(p.p.m.)

C O N TABLE II Analysis (p.p.m.)

C O N A study of the results reveals that the carbon content of heats Aand B increased despite the fact that their oxygen contents decreasedwhereas the carbon and oxygen contents of heat C substantiallydecreased. Heats A and B were not deoxidized in accordance with thisinvention as they involved the injection of pure methane into the melt.On the other hand, heat C was deoxidized in accordance with the methodof this invention as it involved the injection of methane and diluentgas.

It will be apparent to those skilled in the art that the novelprinciples of the invention disclosed herein in connection with specificexamples thereof will suggest various other modifications andapplications of the same. It is accordingly desired that in construingthe breadth of the appended claims they shall not be limited to thespecific examples of the invention described herein.

I claim:

1. A method of deoxidizing molten steel and controlling its final oxygencontent while maintaining a carbon level about equal to or lower thanthe carbon level which Was present prior to deoxidizing, which comprisesthe steps of: analyzing molten steel to determine its oxygen content;introducing hydrocarbon deoxidizer and diluent gas at an averageinjection rate of at least 20 cu. ft. per hour into a vessel containingmolten steel at a subatmospheric pressure, said hydrocarbon deoxidizerreacting with oxygen within said steel to form gaseous carbon compoundswhich exit from said vessel; determining the efiect of the hydrocarbondeoxidizer and diluent gas upon the carbon content of the steel;controlling the proportion of hydrocarbon deoxidizer to diluent gas sothat the average rate of carbon leaving said vessel is about equal to orgreater than the average rate of carbon being introduced into saidvessel; and discontinuing said introduction of hydrocarbon deoxidizerand diluent gas into said vessel after a predetermined amount of oxygenhas been removed from said steel.

2. A method according to claim 1 wherein said introducing of hydrocarbondeoxidizer and diluent gas into 6 said vessel containing molten steelcomprises the step of blowing hydrocarbon deoxidizer and diluent gasinto said molten steel.

3. A method according to claim 1 wherein said introducing of hydrocarbondeoxidizer and diluent gas into said vessel containing molten steelcomprises the step of blowing hydrocarbon deoxidizer and diluent gasonto said molten steel.

4. A method according to claim 1 wherein said molten steel is a secondmelt and wherein said determining of the eifect of said hydrocarbondeoxidizer and diluent gas upon the carbon content of said melt,comprises the steps of: analyzing a first melt contained within a vesselto determine its carbon content; deoxidizing said first melt withhydrocarbon deoxidizer; calculating the amount of carbon introduced intosaid vessel containing said first melt; analyzing said deoxidized firstmelt to determine its carbon content; and calculating the efliciency ofsaid introduced carbon in deoxidizing said first melt.

5. A method according to claim 1 wherein said determining of the efiectof said hydrocarbon deoxidizer and diluent gas upon the carbon contentof said steel, comprises the steps of: calculating the rate at whichcarbon is introduced into said vessel; and calculating the rate at whichcarbon leaves said vessel.

6. A method according to claim 5 including the step of analyzing thegases exiting from said vessel.

7. A method according to claim 1 wherein said average injection rate isat least 30 cu. ft. per hour.

8. A method according to claim 1 wherein said hydrocarbon deoxidizer iscomprised of methane.

9. A method according to claim 8 wherein said diluent gas is comprisedof argon.

10. A method according to claim 1 wherein said hydrocarbon deoxidizer isa liquid and including the steps of atomizing and mixing said liquidhydrocarbon deoxidizer into said diluent gas.

References Cited UNITED STATES PATENTS 2,991,684 7/1961 Wever et al. -23X 3,403,090 9/1968 Tajiri et al. 204- S 3,529,955 9/ 1970 Themelis 75-603,623,863 11/ 1971 Henderson et al. 75-76 3,627,510 12/1971 Vogt et a175-76 411,205 9/ 1889 Slocum 75-59 X 482,001 9/1892 Brazelle 75-591,783,726 12/1930 Lappe 75-59 X 3,046,107 7/ 1962 Nelson 75-59 3,169,0582/1965 Nelson 75-59 X 3,252,790 5/1966 Krivsky 75-60 3,427,151 2/ 1969Koudelka 75-59 OTHER REFERENCES G. R. Fitterer, C. D. Cassley, V.Vierbicky: The Rapid Determination of Oxygen in Commercial Steel withthe Solid Electrolyte Probe, Journal of Metals, June 1968, pp. 74-76.

L. DIEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant ExaminerUS. Cl. X.R. 75-59

